Inventory control system

ABSTRACT

An inventory control system is described which includes anti-collision, radio frequency identification apparatus or tags affixed to each item of an inventory and including a unique code, an interrogator/reader that generates a field of sufficient range to activate each tag associated with each item of the inventory in communication with a computer. The computer includes a list comprising an identifier for each item in the inventory, the code for the tag associated with each item in the inventory and an item status where the status indicates whether the item is present or absent.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/180,524, filed 8 Apr. 2002, which is a continuation of Ser. No.09/329,860 filed 11 Nov. 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inventory control system where eachitem of the inventory is tagged with an anti-collision, radio frequencyidentification apparatus, and the tags are interrogated and read by aninterrogator/reader.

More particularly, the present invention relates to an inventory controlsystem where an interrogator/reader supplies energy to ananti-collision, radio frequency identification apparatus or tag affixedto each item of the inventory and having a unique programmableidentifier and a digital processing unit which stores data sufficient toassociate a given tag identifier with a particular item of inventory andis capable of identifying inventory items outside of the range of thefield generated by the interrogator/reader.

2. Description of the Related Art

Various inventory control systems have been described and implemented inthe market place such as systems for keeping track of items used ingames or sports such as golf, skiing or the like or for keeping track ofchildren or any other item or animal in a given situation. Such systemsare described in various United States patents including: U.S. Pat. Nos.5,870,023; 5,844,483; 5,841;352; 5,757,271; 5,689,240; 5,661,460;5,646,593; 5,565,845; 5,557,259; 5,525,967; 5,493,274; 5,289,163;5,028,909; 5,001,461; 4,973,944; 4,922,229; 4,899,135; 4,489,314;4,535,322; and 4,042,918, incorporated herein by reference.

Although such systems provide some tracking and protection against lossof an item, such systems do not provide efficient multi-item tracking,loss or misplacement protection. Thus, there is a need in the art for asystem which can identify and keep track of multiple items and alert auser when items are lost or misplaced.

SUMMARY OF THE INVENTION

The present invention provides an inventory control system for tagging,identifying and tracking items of an inventory so that accurateinventory accounting can be performed, especially after activitiesinvolving use of items of the inventory. The system includes a digitalprocessing unit (DPU). The DPU includes at least a processing unit, amemory, an operating system, communication hardware and software, and aperipheral for interaction with a user. The DPU also includes a list ortable having an entry for each item in the inventory and comprises atleast a unique item identifier (name), a unique item modulation code,and an item status indicator. The system also includes a radiofrequency, identification (RFID) apparatus removably or permanentlyaffixed to each item in the inventory and encoded with that item'sunique modulation code. The system further includes aninterrogator/reader in communication with the DPU that causes any RFIDapparatus within a range of the interrogator/reader to transmit a signalmodulated by its unique code. The interrogator/reader receives thesignal and forwards the code to the DPU which updates the file so thatthe status indicator, flag or field of the item associated with thetransmitting RFID apparatus is set to a “present” condition (i.e., theitem is within the range of interrogator/reader); otherwise the statusindicator is set to an “absent” condition (i.e., the item is outside ofthe range of the interrogator/reader). Optionally, the operating systemincludes a user interface so that the user can interact with the DPU viathe peripheral. The user interaction would include at least requesting acurrent inventory status check or the like.

The present invention also provides a method for tagging, identifying,tracking and notifying a user when an item in an inventory is notreturned to its pre-use place or designated location after completion ofa given activity involving at least one item in the inventory. Themethod includes tagging each item of the inventory with a radiofrequency, identification (RFID) apparatus encoded with an item specificcode and storing the code in an item indexed list in a digitalprocessing unit (DPU) where the list includes a unique item identifier,the item specific code and an item status flag. Once tagged, aninterrogator/reader causes each item RFID to emit a periodic signalwhenever the RFID is within a range of a radio frequency (RF) fieldgenerated by the interrogator/reader. A received signal causes theinterrogator/reader to send the code embedded in the signal to the DPUwhich sets the item status indicator flag to a condition indicating theitem is within the field, e.g., to a present condition. When an item ispositioned outside of the range of the RF field generated by theinterrogator/reader, then the RFID stops transmitting its signal, theinterrogator/reader stops receiving the signal and stops sending thecode to the DPU. The DPU, then, sets the item status indicator flag to acondition indicating that the item is outside the field, e.g., an absentcondition. When the item is returned to a position within the range ofthe interrogator/reader, the RFID is activated and begins emitting itssignal again, which is received by the interrogator/reader. Theinterrogator/reader then sends the code to the DPU which in turn setsthe item status indicator flag back to a present condition. Uponcompletion of the activity or task, the DPU can be polled by a user todetermine whether any items are missing and the identity of any missingitems. The missing items can then be sought, marked missing or deletedfrom the list. Alternatively, the DPU can alert the user of a missingitem condition by beeping, flashing or some other suitable signal.

DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdetailed description together with the appended illustrative drawings inwhich like elements are numbered the same:

FIG. 1A depicts a preferred embodiment of an inventory control system ofthe present invention;

FIG. 1B depicts a preferred configuration of a digital processing unitused in the inventory control systems of the present invention;

FIG. 2 is an alternative preferred embodiment of a golfbag inventorysystem of the present invention;

FIG. 3 is an alternative preferred embodiment of the present systemlocated in an hospital operating room;

FIG. 4 is an alternative preferred embodiment of the present system islocated in a pasture; and

FIG. 5 is an alternative preferred embodiment of the present systemlocated in a children's play area.

DETAILED DESCRIPTION OF THE INVENTION

The inventor has found that an efficient and effective system forkeeping track of items in an inventory can be designed by affixing toeach item in the inventory an anti-collision, radio frequency,identification (ACRFID) apparatus so that a digital processing unit(DPU) can determine when an item is inside or outside a range of a fieldgenerated by an interrogator/reader apparatus. The inventory cancomprise a plurality of items such as books, children, livestock, toys,equipment, sports equipment such as golf equipment, soccer equipment,baseball equipment, fishing equipment, etc., parts of an apparatus or anarticle, medical equipment used in operating rooms, items used to repairan apparatus or an article, files, or any other item in a given localeor location.

Broadly, the system of the present invention includes a plurality ofradio frequency identification (RFID) apparatus, and preferably, RFIDwith anti-collision capabilities removably or permanently affixed toeach item or animal of an inventory, where each apparatus is capable ofbeing encoded with a unique identification code. The system alsoincludes an interrogator/reader designed to generate an RF field capableof activating the RFID's. Each RFID within range of the RF field emits asignal modulated by its unique code. The system further includes a DPUin communication with the interrogator/reader. The DPU is designed toreceive unique codes and modify an item status field of a list includingat least a unique item identifier, its associated unique code of theRFID affixed to the item, and an item status indicator, e.g., a flag orfield. When an item of the inventory is within the range of the fieldgenerated by the interrogator/reader, then the item's RFID transmits asignal modulated by the code. The interrogator/reader receives thesignal, retrieves the code and sends it to the DPU. The DPU then setsthe item's status flag to a “present” or “TRUE” condition. If an item ismoved of a location outside of the range of the interrogator/reader,then the DPU will set the item's status flag to an “absent” or “FALSE”condition due either to a loss of signal or due to a timeout event.

The method of present invention implemented on a DPU includes affixingto each item or animal of an inventory an RFID encoded with a uniquecode, storing on the DPU in a list including at least a unique item oranimal descriptor or identifier, the unique code associated with itsaffixed RFID and a status flag indicating whether the item or animal isinside or outside a range of an interrogator/reader associated with theDPU. When an item is moved to a location outside the range of theinterrogator/reader, the DPU will change the item's status from a“present” or “TRUE” condition to an “absent” or “FALSE” condition. TheDPU can then be polled at any time to determine the status of all taggeditems or animals within the inventory or plurality. Alternatively, theDPU can change to an “alert” condition if any item in the inventory isoutside the interrogator/reader range and provide a corresponding“alert” indication.

One preferred system of the present invention is a golf equipmenttracking system, where a golf bag includes a DPU, aninterrogator/reader, and a plurality of golf items such as clubs,gloves, or the like having affixed or attached an ACRFID encoded with aunique code. Thus, when a club is removed from the bag, the club willstop transmitting a signal to the interrogator/reader associated withthe DPU. When the signal stops, the DPU will update a status of the clubto evidence that the club has been removed. If the club is returned,then the field generated by the interrogator will cause the tag toresume emitting a signal containing its unique code, the interrogatorwill receive the signal and the DPU will update the status of the clubto indicate that the missing club has been return to the bag. At anytime, the golfer can request the DPU to identify any golf equipment thatdoes not have its status in a “present” condition, i.e., identify theequipment that has not been returned.

Alternatively, the software implemented in the DPU can indicate an“alert” condition if a club is removed from the bag and is not returnedafter some specified period of time. Thus, the DPU software can includeroutines that time/date stamp a field associated with items in theinventory when an item is moved to a location outside of the activatingfield generated by the interrogator and the item's tag stopstransmitting its signal. The routines will then check the time/datestamp for a time off condition evidencing that the item has not beenreturned within a specified period of time. Alternatively, the softwarecan include timer routines that start a timer when communication with atag ceases because the item is now located outside of the activatingfield. The timer will be set to some specified period and a time outwill indicate that the item has not been returned. In either event, atime out will cause the software to place the system in an “alert”condition. Once the item is returned and its tag is in communicationwith the interrogator/reader, then the time keeping routines willdiscontinue time monitoring or disable the timer, set its status fieldto a “present” condition and disable the “alert” condition.

Another preferred embodiment of the present invention is a medicalequipment tracking system for use in operating rooms. This systemincludes a DPU, an interrogator/reader in communication with the DPU,and a plurality of medical equipment such as scalpels, otherinstruments, sponges, or the like, each having affixed or attached anACRFID tag encoded with a unique code. When the tagged items are withinan activation zone generated by the interrogator/reader, the tags willtransmit a signal modulated by its unique code and the DPU will set theitem's status to a condition evidencing its presence within that zone.When an item such as a sponge is removed from the zone, then the item'stag will stop transmitting its signal causing the DPU to update itsstatus indicating that the items has been moved to a location outside ofthe zone. If the item is returned to a location within the zone, thenthe interrogator will cause its tag to resume transmitting its signalcontaining its unique code and the DPU will update its status indicatingthat the item is within the zone. At any time, a medical serviceprovider can request the DPU to identify any equipment that does nothave its status in a present condition, i.e., identify the items thathave not been returned to a location within the zone.

Suitable ACRFID tags include, without limitation, tags manufactured byMicrochip Technology Inc. (www.microchip.comwww.microchip.com) includingMCRF355 tags or the like as well as any other field activated apparatusthat will transmit a signal modulated with a unique modulation code. Theinterrogator and tag can be used to provide inventory control systemswithin the scope of the present invention that are capable of activatinga large number of tags within a given space. The antenna array on thetag and the strength of the field generated by the interrogator can beadjusted by those skilled in the art to increase or decrease a zonewithin which a “TRUE” or “present” condition will be indicated. Forexample, a larger antenna size and a stronger field will increase thesize of the zone within which the tags will be activated by the field.Correspondingly, a smaller antenna size and weaker field strength willreduce the size of the zone within which the tags will be activated bythe field.

By controlling the antenna size associated with the tags and theinterrogator field strength, the size of the zone within which the tagswill be activated can be controlled to a desired degree of accuracy andprecision. The specified tags have an upper limit of several tens offeet. Anti-collision is generally performed by software and hardware ofthe tag which delays signal transmission whenever the hardware andsoftware of the tag senses transmission by another tag. Thus, the tagspreferably send only periodic signals to the interrogator/reader in sucha manner that the reader only receives a signal transmission during aspecified period of time. Thus, these tags generally transmit only aperiodic signal. Because these tags generally transmit onlyperiodically, the tracking software will be designed to expect a signalfrom a tag at some set interval. If a tag fails to transmit a signalwithin the interval, then the software can immediately set an alert oralarm condition (beep, flash, buzz, hum or the like), can wait a givenlength of time before going into an alarm condition or simply note thelack to transmission and await user polling. For child care inventorysystem, immediate alert or alarm setting will be preferred. Immediatealarms will also generally be preferred for store setting or othersetting where immediate response to removed items is required.

Tags designed to function in an anti-collision manner have programmablememory on the tag. The programmable memory includes at least an addressfor a unique code that will be used to modulate the signal transmittedby the tag. The signal is generally in the tens of megahertz range andpreferably in the 10 to 15 MHz range and especially in the 12 to 14 MHzrange and most preferred between about 13 and about 14 MHz. The size ofthe address for the code will determine the number of tags that can bepolled simultaneously by a single interrogator. The tags from Microchiphave a 15 bit code address memory allowing 2¹⁵ (40,728) tags to besimultaneously polled. Generally, the address is at least 7 bits inlength, and preferably at least 15 bits in length. Of course, tags withsmaller or larger memory addresses will allow smaller and largerinventories to be polled.

Suitable DPUs include, without limitation, a computer containing aprocessing unit, a memory, at least one communication protocol supportedby associated hardware and software, at least one peripheral, anoperating system and a filed creation, editing and management systemsuch as a database program, a word processing program, a spreadsheetprogram or the like.

Referring now to FIG. 1A, a preferred embodiment of an inventory controlsystem of the present invention 100 generally includes an inventory 102of items 104A located inside and items 104B located outside of anactivation zone or field 106 generated by an interrogator/reader 108 incommunication with a DPU 110. Each item 104 is tagged with a fieldactivated tag 112 having a unique modulation code 114.

A preferred DPU 110 for use in the present invention is depictedgenerally in FIG. 1B. The DPU 110 includes a processing unit 116, amemory 118, communication hardware 120, and a peripheral 122 forinterfacing with a user. The DPU 110 further includes an operatingsystem 124, preferably a windowing operating system, a user interface126, preferably a graphics user interface (GUI) and communicationsoftware 128. The DPU 110 further includes word or list handlingsoftware 130 and a list 132 comprising an identifier 134 for each item104 of the inventory 102, the unique code 114 of its associated tag 112and a status field 136 which indicates whether a particular inventoryitem 104 is inside or outside of the field 106 generated by theinterrogator/reader 108.

For the items 104A inside the field 106, their status field 134 willindicate that the item is present. If the status field 136 is simply atrue/false flag, then a TRUE condition of the status field 136 canindicate inside or outside the field 106, while a FALSE condition canrepresent the opposite. Thus, the status field 136 associated with eachitem 104A would be in a TRUE condition, while the status field 136associated withe each item 104B would be in a FALSE condition. At anytime, the user can enter a command through the peripheral 122 using theuser interface 126 in conjunction with the operating system 124 todetermine the status of each item in the inventory.

If all of the items 104 originally start out within the field 106, thentheir status fields 136 will all be in a TRUE condition. After a givenactivity involving the items 104, some items 104B may not be returned toa location within the field 106 and the status field 136 for those itemswould be in a FALSE condition. If the inventory is then polled either bythe user or the DPU 110, then the user or the DPU 110 can determinewhich items have a FALSE condition, and, are, therefore, not within thefield 106 generated by the interrogator/reader 108.

Additionally, the DPU 110 can sound an alarm if any item in theinventory is not returned to a location within the field 106 after agiven time after its removal or after the commencement of a given taskor activity. If the inventory includes items used in a medicalprocedure, then the DPU 110 can be polled after the completion ofsurgery or after the estimated time the surgery should take, and missingitems of inventory can be located and returned to their appropriateplace.

Moreover, the DPU 110 can include additional hardware and software toset and stop timers or elapsed time indicators automatically based onactivity levels in a surrounding or start/stop activities of certainapparatus used in a given task or activity.

Referring now to FIG. 2, a preferred embodiment of a golf bag inventorysystem 200 generally includes a bag 202 having associated therewith aDPU 204, an interrogator/reader 206 in communication with the DPU 204,and a power source or supply 208 for supplying electrical power to theDPU 204 and the interrogator 206. Preferably, the interrogator/reader206 has a field range sufficient to activate tags on items locatedinside the golf bag 202, but not tags on items located outside the bag202. The power supply 208 is generally a battery, with lithium batteriesbeing preferred. As in the system of FIG. 1B, the DPU 204 generallyincludes at least a processor, a memory, a peripheral, communicationhardware, an operating system, a file processing routine(s), andcommunication software. The bag 202 also includes a plurality of clubs210; each club 210 having affixed thereto an ACRFID tag 212 encoded witha unique transmission modulation code, where each tag 212 generates asignal modulated by its unique code when the tag 212 is within the rangeof the field generated by the interrogator/reader 206.

The DPU 204, the interrogator 206 and the power supply 208 arepreferably housed in a housing 214 which can be a small shock resistantbox, card or other similar housing removably attached, for example, to acarrying strap 216 of the bag 202 by an attachment strap 217.Preferably, each tag 212 is positioned on each club 210 so that the eachtag 212 will be located in an interior portion 218 of the bag 202 whenthe clubs 210 are place properly within the bag 202. One particularlypreferred the tag placement is to position each tag 212 at or near agrip 220 of each club 210. However, the tags can be affixed to anyportion of the club. The inventor has also found that with items thathave substantial metal components, the metal to the item can be used toincrease antenna dimension without increasing the actual size of theantenna associated with the tag itself.

Preferably, the interrogator/reader 206 generates a field 222 sufficientto activate all tags on all items in the golf bag 202 or very near thebag. Because the control system associated with a golf bag is designedto notify golfers when equipment such as clubs are not placed back intothe bag after use, the field 222 associated with the interrogator 206 ispreferably of a relatively limited range. Generally, the field 222should be able to activate tags 212 within about a volume have a radiusof about 2 feet or less centered about the interrogator 206 andpreferably a radius of about 1.5 feet or less and especially a radius ofabout 1 foot or less. Again, the field strength needed to activate tagsin a given volume will depend on the power of the RF field generated bythe interrogator and the size of the receiving antenna associated withthe tag. As the RF power increases the size of the antenna associatedwith the tags can decrease and vis-a-versa.

Alternatively, the DPU, the interrogator, and the power supply can behoused in a compartment within the bag. It should also be recognizedthat the DPU, interrogator and power supply can be a single ormulti-piece device and can be relatively small in size. The DPU isgenerally includes a relatively modest processor and memory; virtuallyany digital processing unit can be used.

Referring now to FIG. 3, another preferred embodiment of the presentsystem is shown located in an operating room 300 which includes anequipment table 302 and an operating table 304. Associated with theequipment table 302 is a DPU 306, an interrogator/reader 308 incommunication with the DPU 306, and a power source or supply 310 forsupplying electrical power to the DPU 306 and the interrogator 308. Thepower source 310 is generally either an AC outlet or a battery, with anAC outlet being preferred. As shown in FIG. 1B, the DPU 306 generallywill include at least a processor, a memory, a peripheral, communicationhardware, an operating system, a word or list processing routine orroutines and communication software (not shown). Preferably, theinterrogator/reader 308 will have a field range sufficient to activateall tagged items on the equipment table 302, but insufficient toactivate items activated items not on the equipment table 302. The sizeof the field needed will be related to the size of the table for whichthe inventory control system is designed. Preferably, the field shouldhave a range somewhat smaller than the size of the equipment table 302to ensure that equipment is returned to a position within the fieldassociated with the equipment table 302 after an operation.

The equipment table 302 generally includes a plurality of medical items312 used in operations such as sponges 314, scalpels 316, retractors 318or the like. Each item 312 has affixed thereto an ACRFID tag 320 encodedwith a unique transmission modulation code, whereby each tag 320generates a signal modulated by its unique code when the tag 320 iswithin the range of the interrogator/reader 308.

Referring now to FIG. 4, another preferred embodiment of the presentsystem is shown conjunction with an animal containment area such as in apasture 400 including a DPU 402, interrogator/reader 404 incommunication with the DPU 402, and a power source 406 for supplyingelectrical power to the DPU 402 and the interrogator 404, all generallylocated in a vehicle 408. The power source 406 may be a batteryassociated with the starting system of the vehicle 408; however, thepower source can be a separate generator, or a separate battery withlithium batteries being preferred. As in FIG. 1B, the DPU generallyincludes at least a processor, a memory, a peripheral, communicationhardware an operating system, file processing routine and communicationsoftware.

The pasture 400 also includes a plurality of livestock 410, each animal410 having affixed thereto a RFID tag 412 encoded with a uniquetransmission modulation code, whereby each tag 412 generates a signalcontaining its unique code when the tag 412 is within the range of theinterrogator/reader 404. The interrogator/reader 404 generates a fieldhaving range sufficient to activate the tags 412 affixed to animals 410when the vehicle 408 is within a predetermined distance from the animal,with a range of about 5 feet being preferred and a range of about 10foot being particularly preferred.

Of course, the DPU, interrogator and power source can also be positionedon alternative transport means, for example, a horse or a helicopter.The computer software in this preferred embodiment would be programmeddifferently and would not be designed to indicate that all animals werein the pasture. Thus, the software could be programmed so that,initially, all the status fields associated with the animals would be ina FALSE condition. The vehicle would then travel through the pasture asthe vehicle passed near an animal, its tag would be activated, and thesoftware would change the status to TRUE. The DPU could also includedata on each animal to aid the driver in locating and identifyinganimals.

Alternatively, the DPU, interrogator and power source can be associatedwith a gate, entrance or exit to a pasture or corral. In such anembodiment, the computer software would change the status of an animalto a present or TRUE condition when the animal passed through the gate,entrance or exit.

Referring to FIG. 5, yet another preferred embodiment of the presentsystem is shown located at each entrance or exit 500 to a children'splay area 502 which includes a DPU 504, an interrogator/reader 506 incommunication with the DPU 504, and a power source 508 for supplyingelectrical power to the DPU 504 and the interrogator 506. The powersource 508 is preferably either a power supply plugged into an AC plugor a battery, with lithium batteries being preferred. As in shown inFIG. 1B, the DPU includes at least a processor, a memory, a peripheral,communication hardware an operating system, file processing routine andcommunication software. The area 502 will periodically include aplurality of children 510; each child 510 wearing an RFID tag 512encoded with a unique transmission modulation code, whereby each tag 512generates a signal containing its unique code when the tag 512 is withinthe range of the interrogator/reader 506. The interrogator/reader 506has a field range sufficient to activate a tag 512 passing through theentrance or exit 500. In this embodiment, the software is designed totrigger an alarm whenever a child 510 passes within range of the fieldgenerated by the interrogator 506. The alarm may serve to notifyattendants that a child is attempting to leave the area without havingits tag removed.

Although the invention has been disclosed with reference to itsalternative preferred embodiments, from reading this description, thoseof skill in the art may appreciate changes and modification that may bemade which do not depart from the scope and spirit of the invention asdescribed above and claimed hereafter.

1. An inventory control apparatus comprising: a computer including: a processing unit for executing instructions; a memory in electrical communication with the processing unit; a peripheral for interfacing with a user; communication hardware and software for wireless and non-wireless communication; an operating system; a list processing software; and a list implemented therein comprising: a unique item identifier for each item in an inventory; a unique item code; and an item status; an anti-collision, field activated apparatus affixed to each item in the inventory; and an interrogator/reader in communication with the computer which generates a field capable of causing each field activated apparatus within a range of the generated field to transmit a signal containing its unique code; and where the status of each item activated by the interrogator/reader is set to a first state indicating that the item is inside the range of the generated field, while the status of all other items is set to a second state indicating that the item is outside of the range of the generated field and where each anti-collision, field activated apparatus transmits a signal when the anti-collision, field activated apparatus is within the range of the generated field only when no other anti-collision, field activated apparatus is transmitting a signal.
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 21. An inventory control apparatus comprising: a computer including: a list implemented therein comprising: a unique item identifier for each item in an inventory; a unique item code; and an item status; an anti-collision, field activated apparatus affixed to each item in the inventory and including a unique code corresponding to the unique item code; and a reader in communication with the computer and including a generated field capable of activating each field activated apparatus within a range of the field, where the apparatuses within the range of the field transmit a signal containing their unique code setting the status of each corresponding item to a first state indicating that the item is inside the range of the generated field, while the status of all other items is set to a second state indicating that the item is outside of the range of the generated field and where each anti-collision, field activated apparatus transmits a signal when the anti-collision, field activated apparatus is within the range of the generated field only when no other anti-collision, field activated apparatus is transmitting a signal.
 22. The apparatus of claim 1, wherein each field activated apparatus transmits a periodic signal only when no other field activated apparatus transmits a periodic signal.
 23. The apparatus of claim 1, wherein the items in the inventory are medical items associated with a surgery room.
 24. The apparatus of claim 1, wherein the items in the inventory are children.
 25. The apparatus of claim 1, wherein the items in the inventory are animals.
 26. The apparatus of claim 21, wherein each field activated apparatus transmits a periodic signal only when no other field activated apparatus transmits a periodic signal.
 27. The apparatus of claim 21, wherein the items in the inventory are medical items associated with a surgery room.
 28. The apparatus of claim 21, wherein the items in the inventory are children.
 29. The apparatus of claim 21, wherein the items in the inventory are animals. 